In the past, according to MPEG (Moving Picture Experts Group) AAC (Advanced sound Coding) (ISO/IEC14496-3:2001) multi-channel sound encoding technology, auxiliary information such as downmix and DRC (Dinamic Range Compression) is recorded in a bitstream, and a reproducing side can use the auxiliary information depending on the environment (for example, see Non-patent Document 1).
By using such auxiliary information, the reproducing side can downmix a sound signal and control the volume to obtain a more appropriate level by DRC.    Non-patent Document 1: Information technology Coding of audiovisual objects Part 3: Audio (ISO/IEC 14496-3:2001)